11 U.S. Cities With the Worst Air Pollution

The good news: Air has gotten cleaner in recent years. Now the bad: More than half of people in the U.S. still breathe air dirty enough to cause health problems, according to the American Lung Association.

Better air quality

Air has gotten cleaner in recent years. But more than half of people in the U.S. still breathe air dirty enough to cause health problems, according to an American Lung Association (ALA) report.

"There is always going to be a dirtiest city," says Joel Kaufman, MD, director of the Occupational and Environmental Medicine Program at the University of Washington, in Seattle.

Here are the 11 cities (there was a tie for 10th place) with the highest levels of fine particulates, based on annual average. Particulates are tiny (2.5 microns or smaller) and get deep in the lungs, causing health problems.